The present invention relates to an electrode tool for electrochemical machining, and to a method of manufacturing the electrode tool. More specifically, the present invention relates to an electrode tool that is capable of performing electrochemical machining of dynamic pressure generating grooves in fluid bearings with a high degree of precision over long periods of time.
A dynamic groove machining device is utilized to form dynamic grooves on the surface of a work piece such as a fluid bearing. The dynamic grooves generate dynamic pressure on bearing fluid located between the bearing and a shaft to support the shaft within the bearing. As shown in FIG. 1, a conventional electrode tool for dynamic groove machining includes an electrode substrate 1 and a nonconductive insulating film 2 formed by a well known resist method on the surface of the electrode substrate 1. In this type of electrode tool, the finer the conductive pattern 3, the weaker the strength of adhesion between the electrode substrate 1 and the insulating film 2, and the higher the incidence of peeling of the insulating film 2.
The formed grooves correspond to an exposed pattern of conductive areas on the electrode tool. Another known type of electrode tool has a substrate covered in a region outside the aforementioned exposed pattern by an insulating resin layer. This insulating resin layer is formed by adhering and baking fine resin particles onto the substrate. Furthermore, another known dynamic groove machining device includes an electrode tool on which a resin sheet with holes preformed in the dynamic groove pattern to be machined is secured to the surface of the electrode substrate.
Moreover, in an electrode tool for electrochemical machining of fine surface shapes, since the insulating film of the regions outside the machining pattern is thin, the strength of adhesion of the nonconductive insulating resin to the substrate is typically weak. As a result, the insulating film tends to peel off due to the effects of the electrolyte solution used in the electrochemical machining process. This is because, in many cases, the nonconductive resin used for the insulating film is cured by ultraviolet rays, heat or the like, and its adhesion to the conductive substrate used in electrode tools is generally low.
In addition, since electrochemical machining of such fine surface shapes is performed with a narrow machining gap set between the electrode tool and the work piece, the insulating film is exposed to substantial shear forces from the flow of the electrolyte solution. When peeling of the insulating film occurs, it becomes impossible to accurately transfer the machining pattern to the work piece. Furthermore, the machining gap between the electrode tool and the work piece tends to clog with peeled off pieces of the insulating film. This clogging partially obstructs the flow of electrolyte solution, causing defects of the machined shapes in the work piece, and therefore lowering the yield of resulting products, such as dynamic bearings or the like, in which the work pieces are used as components. Moreover, the above-mentioned clogging can cause electrical shorts that damage both the electrode tool and the work piece.
To avoid the above-mentioned clogging problems, another known electrode tool for electrochemical machining is known in which an electrode includes a conductive area formed in a specific pattern on the surface of a conductive substrate. The electrode and a work piece, on the surface of which depressions are to be formed, are immersed facing one another into an electrolyte solution. The work piece and electrode are connected respectively to the positive pole and negative pole of a machining power supply, and current is passed through them to form depressions on the work piece surface corresponding to the conductive area pattern of the electrode. An electrodeposition coating film is formed as the insulating film on the regions of the surface of the electrode other than the aforementioned conductive area pattern.
As shown in FIG. 2, another type of known electrode tool has a conductive pattern with an electrode substrate surface defined by lands 3. The lands 3 are formed by groove machining the surface of the electrode substrate 1 and then molding the substrate 1 with insulating resin. The surface of the electrode substrate 1 is mechanically polished to expose the lands of the conductive pattern. The insulating film 2 is then filled into cut-away depressions in the substrate 1, the surface of which is substantially flat.
However, while there is little susceptibility to the effect of forces accompanying the flow of electrolyte solution in such an electrode tool configuration, the electrolyte solution gradually penetrates the boundary between the insulating film 2 and the conductive substrate 1, resulting in the insulating film 2 ultimately peeling off.
In yet a third type of known electrode tool for electrochemical machining, depressions are formed in those portions of the substrate of the electrode tool that face the work piece and that do not correspond to the dynamic groove pattern. Nonconductive insulating film is provided in the depressions, and the portions of the substrate not covered by the insulating film are formed into an exposed electrode pattern. The surface of the electrode pattern is formed so as to be flush with the surface of the nonconductive insulating film. Therefore, even if the machining gap is narrowed in order to improve transfer precision, there will be no retention of electrolytic byproducts generated by the machining or of heated electrolyte solution, and the desired electrolysis conditions can therefore be maintained. Furthermore, there is no clogging of peeled off pieces of nonconductive insulating film due to collision of electrolytic byproducts with the nonconductive insulating film 2, and drops in electrolyte solution flow rate are prevented.
When the above discussed surfaces on the electrode tool are made flush, concavities and convexities are eliminated, so drops in electrolyte solution flow rate are prevented to a greater extent. When such drops in flow rate are prevented, drops in current density are prevented as well, thereby reducing the surface roughness of the machined surfaces of the work piece. Furthermore, such prevention of drops in current density increases the electrochemical machining rate.
The above electrode tool can be formed with an electrode substrate surface having a conductive pattern defined by lands formed by groove machining. The electrode tool is molded with an insulating resin. The surface of the insulating resin is then mechanically polished to expose the lands of the conductive pattern. The electrode tool allows precise surface machining of work pieces and can withstand prolonged use.
However, it has been determined that when the above conductive pattern surface defined by lands is molded with the insulating resin, and the surface of the resin is mechanically polished, the lands tend to jut outwardly in the groove drop-off direction into an area that should be a trough when the polishing reaches the lands of the electrode substrate surface. This typically results in substantial burring because the electrode substrate, as compared to the insulating resin, has a ductility that is characteristic of metals.